Reproduction apparatus using an endless image-bearing belt

ABSTRACT

Reproduction apparatus utilizing a moving endless belt to receive and transport an electrostatic charge image or a pigment image is provided with belt positioning means of simple form to hold the belt taut and in required alignment. A convexly curved smooth guiding surface, advantageously stationary, has the belt pressed against it and slid over it under tension as the belt enters a processing path in which it receives the image, and edge guides are disposed almost or actually in contact with the opposite side edges of a relaxed portion of the belt approaching the region where the belt is pressed against the slide surface. Belt driving means beyond the processing path keep the belt under tension sufficient to hold it flat and straight in an imagereceiving flight of its travel. In a return region of its path beyond the driving means a length of the belt is kept under substantially no applied tension.

[ Nov. 5, 1974 REPRODUCTION APPARATUS USING AN ENDLESS IMAGE-BEARINGBELT [75] Inventor: Mathias J. J. M. Vola, Sittard, Netherlands [73]Assignee: Oce-van der Grinten, N.V., Venlo,

Netherlands [22] Filed: Jan. 14, 1974 211 App]. N0; 433,052

Related US. Application Data [63] Continuation-in-part of Ser. No.298,783, Oct. 18,

1972, abandoned.

[30] Foreign Application Priority Data 3,536,401 10/1970 Mason et al.355/73 X 3,556,374 l/l97l Demke 226/119 3,646,866 3/1972 Baltazzi et al.355/16 X Primary Examiner-Samuel S. Matthews Assistant Examinerl(ennethC. Hutchison Attorney, Agent, or FirmAlbert C. Johnston 57 ABSTRACTReproduction apparatus utilizing a moving endless belt to receive andtransport an electrostatic charge image or a pigment image is providedwith belt positioning means of simple form to hold the belt taut and inrequired alignment. A convexly curved smooth guiding surface,advantageously stationary, has the belt pressed against it and slid overit under tension as the belt enters a processing path in which itreceives the image, and edge guides are disposed almost or actually incontact with the opposite side edges of a relaxed portion of the beltapproaching the region where the belt is pressed against the slidesurface. Belt driving means beyond the processing path keep the beltunder tension sufficient to hold it flat and straight in animage-receiving flight of its travel. In a return region of its pathbeyond the driving means a length of the belt is kept undersubstantially no applied tension.

19 Claims, 5 Drawing Figures REPRODUCTION APPARATUS USING AN ENDLESSIMAGE-BEARING BELT This is a continuation-in-part of copendingapplication Ser. No. 298,783 filed Oct. 18, 1972 now abandoned.

This invention relates to a reproduction apparatus of the type makinguse of a moving endless belt with means for forming on this belt animage to be transferred to a receiving surface as an elecrostatic chargepattern or pigment pattern.

Many varieties of reproduction apparatus of that type are known. Forinstancez-German Pat. publication No. 1,925,684 (FIG. 2) and U.S. Pat.No. 3,646,866 show an apparatus having a photoconductive endless belt onwhich, by electrostatic charging, exposing and powdering steps, a powderimage is formed which is then transferred to a sheet of receivingmaterial. US. Pat. No. 2,990,278 (FIG. 1) shows an apparatus in which apowder image is formed on a photoconductive drum from which this imageis transferred first to an endless belt and then from the belt to areceiving sheet. US. Pat. No. 3,051,568 (FIG. 5) shows an apparatus inwhich a charge pattern created by exposure on a photoconductive drum istransferred to an endless belt and developed on this belt by powderingit, after which the powder image thus formed is transferred to a sheetof print paper.

In order to produce sharp and blur-free images with this type ofapparatus, it is necessary that theendless belt be kept flat and alignedvery accurately.

It is difficult to keep endless belts in alignment, and

particularly so when the belts are relatively wide, e.g. wider than '20cm. Such belts have a strong tendency to move sideways on the driveand/or guide rollers over which they are moved. This tendency may becaused by any of a variety of factors, such as unevenness in the beltmaterial and/or variations in the parallelism or the friction of thebelt positioning rollers.

Many expedients have been proposed for preventing or correcting lateralmovement of endless belts.

A system frequently used for conveyor belts makes use of a steeringroller the position of which can be changed with the aid of electrical,mechanical or prieumatic actuating means for correction of the path ofthe in the formation of an image, because it'may debase objectionablythe sharpness of the image obtained. Furthermore, the steering rollersystem is quite expensive and complicated, and the required detectionand switching means'are operated so frequently that they have a shortservice life.

Another system for aligning conveyor belts makes use of guide means inthe form of arrests or rollers installed at each side edge of the belt,with which the belt makes contact when it moves sideways so as to causea reactive force urging the belt back into the correct path. Undercertain conditions a favorable result can be obtained with such asystem. On the other hand, the forces exerted on the edges of the beltare relatively great, so that it is not practicable to use a system ofthis kind for positioning thin or delicate belts such as those desirablyused in reproduction apparatus of the type to which the presentinvention relates. The edges of such belts would be damaged byfrictional wear, creasing, or folding, at the locations where thecorrecting forces are exerted upon them. I

' Other means of various sorts are known for aligning narrow conveyorbelts and driving belts, such as the use of special forms of the beltsand of the belt positioning rollers. These may be made trough-shaped,V-shaped, conical shaped, etc. in order to assure the requiredalignment. Such means, however, offer no remedy for the problem ofpositioning the relatively wide belts required in the type of apparatusunder consideration here.

The principal object of the present invention is to provide an effectiveyet technically simple way and means of keeping accurately aligned andtransversely flat an endless image-carrying belt of reproductionapparatus of the type above mentioned.

A further object of the invention is to provide in such apparatus anarrangement for positioning such a belt by which a flight of the beltwill constantly be kept straight and flat in a definite path, for thereception of an image projectedoptically onto its surface.

According to the present invention, these objects are attained by meansof a belt positioning system in which the endless belt is driven at alocation downstream from the processing path wherein the image formingand image transfer are effected, and at a location upstream therefromthe belt is kept pressed constantly in contact with and slid over aguiding surface that does not move at the linear speed of the belt, sothat the length of the belt in said path is heldtaut, while a furtherlength of it'in a belt return region between the driving means and saidsurface is kept relaxed, and in which, at the opposite, sides of thepath of movement of a relaxed portion of the belt approaching the regionwhere the belt is pressed against said surface, edge guides are providedwith which the side edges of the belt are kept almost if not actually incontact.

According to a further feature of the invention, the belt driving meanscomprises a driven roller provided with an elastic sheathwhich engagesthe full width of the belt and is sufficiently soft to be depressible bytension in the belt. The belt is laid upon and held to the surface ofthe elastic roller sheath and by rotation of the roller is drawn fromthe processing path with the force required for sliding it over theguiding surface at the upstream side of that path. It has been foundthat the elastic sheath contributes importantly to the maintenance ofthe desired alignment of the belt. This is believed to be dueto the factthat when irregularities of tension occur in local regions of the widthof the belt, corresponding incremental variations of the effectiveradius, hence of the peripheral belt driving speed, re-

sult in local regions of the elastic sheath so as to counteract thevariations of tension.

Surprisingly it has been found that the invention enables the belt to bemaintained in highly accurate alignment and that, at most, only veryslight forces are exerted on the side edges of the belt when they makecontact with the edge guides. This makes it possible to use very thinand/or delicate belts without risk of damage to them. It has been found,for instance, that belts of less than 1 mm in thickness and made ofdielectric materials (e.g. rubber impregnated cloth, polyester, orpolyolefme), with or without an electrically conductive layer (e.g. avacuum deposited metal coating), as well as belts made of electricallyconductive materials (e.g.

metal, paper, or conductive plastic), with or without a photoconductiveor dielectric coating, can be controlled excellently in this way.

A suitable guiding or slide surface that does not move at the linearspeed of the belt can be provided by use of a roller that rotates withmovement of the belt but at a peripheral velocity lower than the speedof movement of the belt, or by use of a roller that rotates in adirection opposite to the direction of movement of the belt. It has beenfound especially advantageous, however, to utilize a belt guidingsurface that is stationary. Remarkably good results are obtained by theuse of a convexly curved stationary surface, such as a semicyclindricalsurface.

The belt may be kept pressed in contact with the guiding surface bymeans of tension applied to the belt, such as by a tensioning roller, asthe belt is driven and pulled overthe convex guiding surface. Instead ofor in combination with such a tensioning of the belt, the belt may bekept pressed in contact with the guiding surface by means of a web orcloth, with or without soft hairs, flexibly stretched along and againstthe belt upon that surface.

The guiding surface with which the belt is kept in contact may becomposed of any of a variety of materials. The material to be used is,of course, selected with a view to minimizing wear of the belt as aresult of its sliding motion under pressure relative to the guidingsurface. Accordingly, verysmooth surfaces, for instance chromium-platedmetal surfaces or surfaces coated with Teflon (i.e.,polytetrafluoroethylene), are eminently useful.

It has been further found, however, that it is advantageous to make theguiding'surface of a soft hairy or napped material, for instance, offelt or of a mohair or nylon plush. Where there is a risk of dust orother abrasive material (e.g. the remains of a powder image) enteringbetween the guiding surface and the belt, the surface made of hairymaterial will reduce the risk of scratching of the sliding parts byaccepting dust or other loose particles in the spaces between its hairs.

The invention will be further understood from the following detaileddescription and the accompanying drawings of an illustrative embodimentthereof. In the drawings:

FIG. 1 is a schematic longitudinal sectional view of anelectrophotographic copying apparatus provided with an endlessphotoconductive belt in accordance with the invention;

FIG. 2 is a schematic front view, partly in schematic cross section, ofthe same apparatus;

FIG. 3 is a perspective view of the stationary belt aligning means ofthe apparatus of FIGS. 1 and 2;

FIG. 4 is a schematic sectional view of an electrophotographic apparatususing a wide, zigzag-folded endless photoconductive belt; and

conductive coating (e.g. metallized polyester film) to which a layer ofa photoconductive composition (e.g. zinc oxide in an insulating binder)has been applied.

With the aid of flash lamps 3 and 4, mirrors 5 and 6 and objective 7,the image of an original lying on glass plate 2 is projected onto thesurface of belt 1 in a vertically disposed flight la thereof, after thisportion of the belt has been electrostatically charged by means of acorona device at 8. The latent image or charge pattern formed on thephotoconductive belt by the flash exposure is developed into a powderimage by means of a magnetic brush device at 9. The powder image is thentransferred to a sheet 11 of copy paper, fed from a tray 10.Subsequently, the paper sheet carrying the powder image is fed betweenthe rollers 12 and I3 which. being heated by heating elements 14 and 15,fuse the powder image and thus fix it to the paper. The copy thusobtained is finally deposited on the top side of the apparatus by meansof feed rollers 16 and 17. After transfer-v ring the powder image, thebelt 1 is cleaned with the aid of a rotating polishing brush 18 whichremoves any residual powder from the photoconductive belt surface.

The belt 1 is arranged and held in working position and driven at therequired speed by being trained along its upper reaches over the guiderollers 19 and 20 and over a driven roller 22 which drives the beltcontinuously and evenly. Roller 22 is provided with a resilient sheath22a, for example of an elastic rubberhaving a hardness of 50 to 70 Shoreand a thickness of 10 to 20 mm. Along its lower reaches the belt 1 isguided by a roller 21 and thence over a stationary aligning means 23whichis more fully illustrated in FIG. 3. This aligning device consistsof a convexly curved plate 24 of substantially semi-cylindrical. form,the surface of which is constituted by a layer 25 of a soft hairy ornapped material, such as felt, and which is provided with end flanges 26and 27 protruding radially beyond the napped surface. The distancebetween the flanges 26 and 27 is only slightly greater(e.g. 1 mmgreater) than the width of belt 1.

The guide roller 21, in order to avoid damaging the photoconductive beltsurface, may be coated with a soft or napped material such as mohair.

The belt 1 is so arranged that it is held over its entire width in closecontact with the napped surface 25, so that when it is being driven byroller 22 it slides over surface 25 while being pressed evenly againstthis surface. Due to the pressure and resulting friction between thebelt and surface 25 and the driving action of roller 22, the length ofthe belt, including flight la, which extends through the processing pathbetween surface 25 and roller 22 is kept under a certain desiredtension, while the further length of the belt which extends through thereturn path or region beyond roller 22 is relaxed. As a portion of therelaxed length of the belt approaches the region where the belt ispressed against the slide surface 25, it is guided to the desiredposition FIG. 5 is a perspective view of the slide surface and beltpressing means of the apparatus of FIG. 4.

Theapparatus represented schematically in FIGS. 1

and 2 of the drawings is provided with a thin (e.g.-

0.l0.5 mm thick) endless flexible belt 1. This belt is made, forexample. of an electrically conductive material (eg. metal) or of amaterial having an electrically on that surface by the fixed side edgeguides which in the embodiment shown are the flanges 26 and 27. It willbe understood that little force needs be exerted on the side edges oftherelaxed portion of the belt in order I to keep it directed toward andupon the same region of tension maintained in the length of belt 1downstream from surface 25, the flight la of the belt onto which theimage is projected is easily kept perfectly flat and straight asrequired in order to assure that a sharp image of each original exposedon plate 2 may be projected onto the belt surface.

In the region of its return path beyond the driving roller 22 the belt 1need not be tensioned at all, or may be under considerably less tensionthan in the flight la where it receives the projected image-forminglight. The provision of such a freely suspended" region of the belt pathis particularly desirable when the belt is made mainly of nonelasticmaterial such as metal or paper. Thisis also advantageous when a verylong belt is used, in which case a very large part of the belt length,not under tension and substantially constituting a stock ofphotoconductive belt material, may be maintained in the return regionbetween the belt driving roller and the guiding surface over which thebelt is slid into the processing path.

The close contact between the belt and the guiding surface may of coursebe effected by pressing these members together in various ways. Forexample, a web 28 of a woven textile, or cloth, may be stretched overthe belt 1 on surface 25 and held taut with the aid of springs orweights so as to press the belt against said surface. Such a cloth 28working over the entire width of the belt is represented schematicallyby dotted lines in FIG. 1. Instead of such a wide web or cloth, a numberof narrower webs or cloths lying side by side at intervals can of coursealso be used.

FIG. 4 and Fig. 5 are schematic views of an embodiment of the inventionas it is adapted for use in a photocopying apparatus which also makesuse of a photoconductive endless belt pre-folded in zigzag manneraccording to the invention disclosed in US. Pat. application Ser. No.370,680 filed June 18, 1973 as a continuation of application Ser. No.226,413 filed Feb. 15, 1972. As described more particularly in thatapplication, such apparatus is provided with a long endless belt 30 offlexible photoconductive material which, as a result of its previouszigzag folding, is composed of a large number of equally sized panels orsections (e.g. 625 sections, each of approximately 40 cm in length)which are easily disposable one upon another for storage in a compactstack. The belt 30 may have, for example, a width of 25 30 cm and atotal length of approximately 250 meters, and it may be composed, forexample, of a paper weighing about 120 g/m coated with a layer weighingabout g/m of zinc oxide in an insulating binder.

The major part of the belt length is disposed in a stack 31 of beltsections laid on top of each other in" a magazine 32 of the typedisclosed in a copending US. Pat. application Ser. No. 226,870, now US.Pat. No. 3,765,488. A shorter part of the length of belt 30 is guidedfrom the bottom of the magazine over and along a smooth convexly curvedguide, or slide, surface 35 of a stationary belt guiding member 34, fromwhich it passes through a processing path wherein the belt is po-.

When the driving roller 42 is rotated, the belt 40 is driven evenly, forexample at a speed of 15 m/min., and the belt material is drawncorrespondingly from the under side of stack 31, then along and over theaforesaid slide surface 35, thence through the processing path locatedbetween that surface and the nip of the rollers 42 and 43, and thence ina belt return region which in this embodiment comprises the stack 31 ofbelt sections laid one upon another in the magazine.

In order to maintain a desired tension in the belt length drawn throughthe processing path, and also to prevent the belt from flipping" overthe surface 35 at the moments when a belt section between two foldsisdrawn from the magazine 32, the belt is constantly pressed in contactwith surface 35 by means of a holddown means 54. This means (see FIG. 5)comprises a fixed rod 55 having two bands 56 of a napped, or hairy,material (,e.g. bands or webs of nylon plush about 5 cm wide) fastenedto it at one of their ends. The other ends of these bands are fastenedto a loose rod 57 which is pulled under a substantially constant forceby a tension spring 58. The bands 56 are stretched along and over aportion of the belt 30 engaged between them and the convex slide surface35, so that the belt is constantly pressed against that surface with aforce sufficient to keep the belt length being drawn into the processingpath under a desired tension. The magnitude of the pressing forceexerted by the bands 56 can, of course, be varied by adjustment of thetension applied by spring 58.

Due to the driving action of roller 42 and the friction which must beovercome in order to cause the pressed region of the belt to slide overthe guide surface 35, the length of the belt extending between surface35 and roller 42 is held taut, while the remaining length of the belt,being mainly disposed in stack 31 under no tension, is relaxed.

An electrophotographic image is formed on a section of the belt lengthextending through the processing path and is transferred from it to areceiving material by the action of suitable processing meanssuch as acharging unit 44, an exposure system optically exposing a panel of thephotoconductive surface in zone 45 between rollers 37 and 38, adeveloping unit 46, and a transfer unit at 47. Finally, before beingdeposited back into the stack, the belt is acted upon by a cleaning unit48.

As a result of the tension maintained in the belt length extendingbetween surface 35 and the driving roller 42, the folds between thesuccessive belt sections are flattened in that length of the belt sothat it is held in straight runs between the guide rollers in theprocessing path. Accordingly, sharp electrophotographic images can beapplied to these sections of the traveling belt, without anyobjectionable influence being exerted thereon by the folds, and theimages can be transferred effectively to a receiving material 49 broughtinto contact with the belt 30 at the transfer station 47 The portion ofthe belt 30 which has been used to form and transfer an image is laidsection by section onto the top of the stack 31 with the aid ofoscillating movements of roller 43 and of rocking movements of hold-downmeans 50, 51 and 52, as described more particularly in the aforesaid US.Pat. No. 3,756,488. Upon being thus redeposited in the magazine 32, the

stacked sections of the belt are brought laterally to a which lietransverse to the axis of its curvature and guide the edges of the beltpassing to and over it in the manner of the flanges 26 and 27 of H68. 1and 3. Consequently, the successive belt sections leaving the magazinewill always slide to and over the same region of the surface 35, so thatthe belt is kept properly in alignment with the processing units as itis drawn into the processing path.

The edge guide strips 53 and/or those at 35a and 35b apply hardlyanyforce or friction to the side edges of the belt; so the belt can bepassed a great number of times (eg more than 200 times) through theapparatus without showing edge wear, even when it is composed of a quitethin or delicate material such as a coated paper.

The use of two hold-down strips pressing the belt against the curvedslide surfaces is advantageous in that these strips can be arranged topress against marginal regions of the belt that ordinarily do not takepart in the formation of images on the belt; so no damage to the imageforming regions of the belt by undesirable friction is likely to occur.

What is claimed is:

1. ln reproduction apparatus including an endless belt, meanspositioning said belt for movement thereof through a processing path andthence through a region for return to said path and means respectivelyfor forming an image on and transferring the image from a portion of thebelt in said path, the combination which comprises means for driving thebelt engaging said belt at a location downstream from said path betweensaid path and said return region, a slide surface engaged by said beltat a location upstream from said path and over which the belt slides asit is moved by said driving means, means for pressing said belt againstsaid slide surface with a force whereby tension is created in the lengthof the belt extending between said surface and said driving means so asto hold the belt taut in said path, the portion of the belt in saidreturn region being longer than the shortest distance provided for belttravel therein and thus being substantially tensionless therein, andedge guides at the opposite side edges of a tensionless portion of saidbelt approaching said slide surface for guiding the belt constantly overa selected portion of said surface.

2. Apparatus according to claim 1, said driving means comprising adriven roller provided with an elastic sheath which engages the fullwidth of said belt and is sufficiently soft to be depressible locally bylocal variations of tension in the belt.

3. Apparatus according to claim 1, said slide surface being a smoothconvexly curved surface.

4. Apparatus according to claim 1, said slide surface being the surfaceof a roller having means to enable movement of it only at a peripheralspeed different from the speed of the movement of said belt by saiddriving means.

5. Apparatus according to claiml, said slide surface being a smoothsubstantially semi-cylindrical outer surface portion of a stationarybelt guiding member.

6. Apparatus according to claim 5, said surface portion beingconstituted by chromium plated metal.

7. Apparatus according to claim 5, said surface portion beingconstituted by a coating of polytetrafluoroethylene on metal.

8. Apparatus according to claim 5, said surface portion beingconstituted by a layer of a napped material.

9. Apparatus according to claim 1, said slide surface being a smoothouter surface of a substantially semicylindrical portion of a stationarybelt guiding member, said member having thereon flanges which protrudesubstantially radially from opposite ends of said semicylindricalportion and constitute said edge guides.

10. Apparatus according to claim 1, said edge guides being mutuallyspaced apart by a distance not more than about 1 mm. greater than thewidth of said belt.

11. Apparatus according to claim 1., said path com prising a flatexposure zone having a flight of said belt extending therethrough andsaid belt being pressed against said slide surface with sufficient forcethat the tension on said belt in said path holds said belt flightstraight and flat.

12. Apparatus according to claim 1, said pressing means comprising atleast one flexible web of a material having little frictional resistanceto movement of said belt and means holding said at least one flexibleweb stretched under tension against and along said belt over a region ofsaid slide surface.

13. Apparatus according to claim 12, said at least one flexible webhaving a smooth napped surface on the side thereof bearing against saidbelt.

14. In electrophotographic reproduction apparatus including an endlessbelt having a photoconductive surface, means positioning a length ofsaid belt in and for movement through a processing path comprising astraight path part, means for projecting an optical image onto saidsurface in said path part to form an electrostatic image thereon, andmeans positioning a further length of said belt in a belt return regionaway from said path, the combination which comprises belt driving meansincluding a roller engaging said belt downstream from said path androtatable to drive said belt from said path into said return region,said roller having an elastic sheath which engages the full width ofsaid belt and is sufficiently soft to be depressible locally by localvariations of tension in the belt, a stationary convexly curved beltguiding surface engaged by said belt at a location upstream from saidpath and over which the belt slides as it is driven by said roller,means for pressing said belt against said guiding surface withsufficient force that the belt length between said guiding surface andsaidroller is kept taut and is held flat in said straight path part,said further length of said belt being longer than the shortest distanceprovided for belt travel through said return region and thus beingsubstantially tensionless in said return region, said pressing meansincluding at least one flexible web of a material having littlefrictional resistance to movement of said belt and means holding said atleast one flexible web stretched under tension against and along saidbelt .over a region of said guiding surface, and belt edge guides at theopposite side edges of a tensionless portion of said belt approachingsaid pressing means for guiding the belt constantly over a selectedportion of said guiding surface.

15. Apparatus according to claim 14, said at least one flexible webhaving a smooth napped surface on the side thereof bearing against saidbelt.

16. Apparatus according to claim 15, said belt guiding surface being asmooth substantially semicylindrical outer surface portion of astationary belt guiding member, said at least one flexible web having asmooth napped surface on the side thereof bearing against said belt.

17. Apparatus according to claim 16, said surface portion beingconstituted by a layer of napped material.

1. In reproduction apparatus including an endless belt, meanspositioning said belt for movement thereof through a processing path andthence through a region for return to said path and means respectivelyfor forming an image on and transferring the image from a portion of thebelt in said path, the combination which comprises means for driving thebelt engaging said belt at a location downstream from said path betweensaid path and said return region, a slide surface engaged by said beltat a location upstream from said path and over which the belt slides asit is moved by said driving means, means for pressing said belt againstsaid slide surface with a force whereby tension is created in the lengthof the belt extending between said surface and said driving means so asto hold the belt taut in said path, the portion of the belt in saidreturn region being longer than the shortest distance provided for belttravel therein and thus being substantially tensionless therein, andedge guides at the opposite side edges of a tensionless portion of saidbelt approaching said slide surface for guiding the belt constantly overa selected portion of said surface.
 2. Apparatus according to claim 1,said driving means comprising a driven roller provided with an elasticsheath which engages the full width of said belt and is sufficientlysoft to be depressible locally by local variations of tension in thebelt.
 3. Apparatus according to claim 1, said slide surface being asmooth convexly curved surface.
 4. Apparatus according to claim 1, saidslide surface being the surface of a roller having means to enablemovement of it only at a peripheral speed different from the speed ofthe movement of said belt by said driving means.
 5. Apparatus accordingto claim 1, said slide surface being a smooth substantiallysemi-cylindrical outer surface portion of a stationary belt guidingmember.
 6. Apparatus according to claim 5, said surface portion beingconstituted by chromium plated metal.
 7. Apparatus according to claim 5,said surface portion being constituted by a coating ofpolytetrafluoroethylene on metal.
 8. Apparatus according to claim 5,said surface portion being constituted by a layer of a napped material.9. Apparatus according to claim 1, said slide surface being a smoothouter surface of a substantially semi-cylindrical portion of astationary belt guiding member, said member having thereon flanges whichprotrude substantially radially from opposite ends of saidsemi-cylindrical portion and constitute said edge guides.
 10. Apparatusaccording to claim 1, said edge guides being mutually spaced apart by adistance not more than about 1 mm. greater than the width of said belt.11. Apparatus according to claim 1, said path comprising a flat exposurezone having a flight of said belt extending therethrough and said beltbeing pressed against said slide surface with sufficient force that thetension on said belt in said path holds said belt flight straight andflat.
 12. Apparatus according to claim 1, said pressing means comprisingat least one flexible web of a material having little frictionalresistance to movement of said belt and means holding said at least oneflexible web stretched under tension against and along said belt over aregion of said slide surface.
 13. Apparatus according to claim 12, saidat least one flexible web having a smooth napped surface on the sidethereof bearing against said belt.
 14. In electrophotographicreproduction apparatus including an endless belt having aphotoconductive surface, means positioning a length of said belt in andfor movement through a processing path comprising a straight path part,means for projecting an optical image onto said surface in said pathpart to form an electrostatic image thereon, and means positioning afurther length of said belt in a belt return region away from said path,the combination which comprises belt driving means including a rollerengaging said belt downstream from said path and rotatable to drive saidbelt from said path into said return region, said roller having anelastic sheath which engages the full width of said belt and issufficiently soft to be depressible locally by local variations oftension in the belt, a stationary convexly curved belt guiding surfaceengaged by said belt at a location upstream from said path and overwhich the belt slides as it is driven by said roller, means for pressingsaid belt against said guiding surface with sufficient force that thebelt length between said guiding surface and said roller is kept tautand is held flat in said straight path part, said further length of saidbelt being longer than the shortest distance provided for belt travelthrough said return region and thus being substantially tensionless insaid return region, said pressing means including at least one flexibleweb of a material having little frictional resistance to movement ofsaid belt and means holding said at least one flexible web stretchedunder tension against and along said belt over a region of said guidingsurface, and belt edge guides at the opposite side edges of atensionless portion of said belt approaching said pressing means forguiding the belt constantly over a selected portion of said guidingsurface.
 15. Apparatus according to claim 14, said at least one flexibleweb having a smooth napped surface on the side thereof bearing againstsaid belt.
 16. Apparatus according to claim 15, said belt guidingsurface being a smooth substantially semi-cylindrical outer surfaceportion of a stationary belt guiding member, said at least one flexibleweb having a smooth napped surface on the side thereof bearing againstsaid belt.
 17. Apparatus according to claim 16, said surface portionbeing constituted by a layer of napped material.
 18. Apparatus accordingto claim 14, said belt guiding surface being a smooth outer surface of asubstantially semi-cylindrical portion of a stationary belt guidingmember, said member having thereon flanges which protrude substantiallyradially from opposite ends of said semi-cylindrical portion andconstitute said edge guides.
 19. Apparatus according to claim 14, saidsheath being a layer about 10 to 20 mm. thick of a rubber having a Shorehardness of 50* to 70*.